whats up with my Renault Megane 1.4?

I have had loads of problems with my car, only 6 weeks ago having a engine re build as the cam belt went, so had it back and the engine management light never went off, over the past few days, after been driving for less than 10 mins the light flashes and power if lost, like in limp mode, but then the engine rattles shaking the car! When driving the car is under powered but when I stop, eg traffic lights it realy struggles to get going again!
Its a 1.4 dynamique 16 valve 03 plate. Thanks for any help.